Pl. is the written abbreviation of plural — the form of a word that means more than one. Dictionaries and grammar notes use it: child pl. children, analysis pl. analyses. You will also see it in older scholarly notes. It is a label about language, not a word you drop into a sentence. Do not say “pl.” when you mean “more than one”; say plural, or just use the plural form. Though actually pl. is easy to misread, because pl. can also stand for place, plate, or plural in other lists — but in a dictionary entry, pl. almost always means plural. Some books write plur. instead to make that clearer.